Critical strike (often abbreviated as "crit" or "crit strike") refers to 100% bonus physicaldamage (twice your normal damage) that occurs as a result of an attack made with melee or ranged weapons. The chance to critical strike can be viewed in the character pane under melee stats and is affected by agility, critical strike rating, and talents. It is constrated with Critical Strike Rating, which (as of patch 3.0.2) affects melee, ranged and spells. Some classes have talents that increase the amount of bonus damage for critical strikes or talents that increase their critical strike rating. Attacks made by mobs can not result in a blocked critical strike, but this can result on a player attack. It can also be viewed by opening the spell book and mousing over the "attack" ability. Notable about critical strikes is that any non-spell attack on a sitting target will always result with a critical hit(taking into the account that you are more vulnerable sitting or lying down than you are standing face-to-face with your enemy).

This section concerns content that is out-of-date. Reason: Except for level 80 numbers.

Critical Strike Rating is a combat rating found on gear, enchants, and gems. As of Patch 3.0.2, Critical Strike Rating affects melee, ranged, and spells. As of Patch 6.2.3, it takes approximately 17.7 rating to gain 1% critical strike at level 80. Before the stat squish, it took approximately 22.08 rating to increase your critical strike chance by 1% at level 70.

Mobs which are the same level as you always have a 5% chance to crit. A mob's or player's crit chance is modified by the difference between the attacker's Attack Rating and the defender's Defense. The attack rating equals the skill with the currently equipped weapon (WS = Weapon Skill), being level * 5 for mobs and the same for player chars with maximum weapon skill. Each point of AR exceeding the target's Defense will increase chance to crit by 0.04%.

The agility you need to acquire 1% critical hit chance depends on your class and level. Keep in mind everyone has a base crit of 5% from agility. Only agility on gear adds to the base of 5%. Ex. If a level 80 paladin has 161 agility (90 base agility and 71 from gear) the crit chance from agility will be 5% + 71/52.08 = 6.36%.

Both increased hit and crit chance have a kind of "diminishing returns" mechanic built in. A 1% increase of either stat will not increase the total damage output by 1%, but usually by a little less. Consider the following Attack table:

We increased the total damage output by 1% of the base damage. Since the total output is more than 100% of that base damage, the actual increase in total damage output is "only" 0.93%. If the total output is significantly greater than the base damage, this percentage will be smaller, if the total output is below the base damage, the resulting increase will even be higher than 1%.

In principle they are equal. Which is better depends on circumstances. An increase in hit rate will usually yield a constant level of damage, whereas a high crit chance gives a spiky, more random damage distribution over time. An important factor is whether there are procs related to either hits or critical hits. Also, both ratings have caps (which are not fixed to a certain % but rather depend on multiple factors such as hit chance, enemy's level and defense rating). To hit can only be used to counter the base chance to miss (5% for one or two-hand users without any off-hand weapon, and 24% for dualwielders), whereas the crit chance cannot become higher than available space for hit chance. If a dual wielder attacks a target with a high chance to dodge or parry the attacker can lose crit chance because the attack table is full. (See the "Crit Cap" section below.)

For example, a dual wield in a raid, attacking from behind, with a 0 chance to hit, you have a maximum critical chance of 40%. Any additional critical chance increase is wasted. This number might seem high but take into consideration talents, potions, buffs, procs and skills that increase your crit rate to 50% or more, these are wasted.

From a raiding perspective, your miss chance increases by 3% (so 8% and 27% - base of 8% for special attacks) and increasing your Hit to at the very least cover your Special attacks is seen as more beneficial than increasing your Critical chance. After you have hit that level, you can focus more heavily on Critical Chance.

White-damage attacks made by a player with fully-trained weapon skill against a mob 3 levels higher have a fixed 24% chance of being glancing blows.

Therefore, with no +expertise/+hit gear on, and no talents or racial abilities that increase Expertise or the chance to hit, attacks made by a level 80 dual wielder against a level 83 mob have the following table from the front:

From the front, dual-wielding

Result

Chance

Miss

27.0%

Dodge

6.5%

Parry

14%

Glancing Blow

24.0%

Block

6.5%

Combined hit + crit

21%

Attacks against mobs made from the back cannot be blocked or parried (thus, attacking from behind will never trigger "parry haste" on the mob). Losing those values from the chart makes the "combined hit + crit" entry appropriately larger.
Players cannot dodge, parry, or block attacks from the back.

It is not as bad if the same character attacks the same mob from the front with a two-handed weapon:

From the front, not dual-wielding

Result

Chance

Miss

8%

Dodge

6.5%

Parry

14%

Glancing Blow

24.0%

Block

6.5%

Combined hit + crit

40%

This is a 40% + your +hit + 2x your Expertise bonus crit cap.

Finally, consider attacking the mob from the rear, where the mob cannot block or parry:

From the rear, dual-wielding

Result

Chance

Miss

27%

Dodge

6.5%

Parry

0%

Glancing Blow

24.0%

Block

0%

Combined hit + crit

42.5%

From the rear, not dual-wielding

Result

Chance

Miss

8%

Dodge

6.5%

Parry

0%

Glancing Blow

24.0%

Block

0%

Combined hit + crit

61.5%

All these crit caps could have been increased by increasing your Expertise (0.5% increase per point from the front, 0.25% increase per point from the rear) and by +hit (1% increase per percantage increase in your Hit chance).